Temperature and precipitation extreme compound events in Southeastern South America and the associated atmospheric circulation.

RUSTICUCCI MATILDE; TENCER B; BETTOLLI MARIA LAURA

Paris

Conferencia; International Scientific Conference, ?Our Common Future Under Climate Change?; 2015

Compound events consist of the simultaneous or successive occurrence of two or more extreme events, the combination of extreme events with conditions that amplify the impact of the events, or the combination of events that are not individually extreme but can lead to extreme impacts when occurring together (IPCC, 2012). In this paper we analyse the joint occurrence of extreme temperature and heavy precipitation events (simultaneous or lagged by one day) in southern South America during 1961-2000.The study is based on a comprehensive dataset of daily precipitation and daily minimum and maximum temperature observed at meteorological stations of the region and compiled during the CLARIS LPB project. Four different extreme temperature events were defined: warm nights (days) correspond to days with minimum (maximum) temperature exceeding the 90th percentile of the daily distribution; cold nights (days) are days with minimum (maximum) temperature below the 10th percentile. Heavy precipitation events are events with daily rainfall above the 75th percentile of the empirical distribution of rainy days. A compound event is defined when one of the above temperature extremes occurs simultaneously with, preceded or followed by a heavy precipitation event.The existence of a significant statistical relation between these extremes could help to better characterize the uncertainties associated with projections of extreme precipitation events for a future warmer climate. Results show that the probability of occurrence of an intense precipitation increases during or after a warm night, but decreases during a cold night, compared to the expected likelihood of occurrence of this type of events in the absence of a relation between temperature and precipitation extremes. Warm days are usually associated to the occurrence of heavy precipitation events on the same day or the day before, but they rarely occur afterwards. On the contrary, cold days happen more often after an intense rain.In order to characterize the atmospheric circulation during the occurrence of a compound event, we use a synoptic classification developed by Barrucand et al (2014) and based on daily mean fields of geopotential height at 500hPa from the NCEP2 reanalysis. The associated circulation during a compound event of warm nights or warm days and heavy precipitation shows a trough over the Pacific Ocean and a cold front over the continent that lead to warm and wet air advected to the east of the region of study. Cold days and heavy precipitation events in the southwestern part of the domain of study are usually characterised by a positive anomaly of geopotential height at the southern part of the continent associated with an eastern anomaly over the region.